Air preheater and method for preventing corrosion and blockage of the same

ABSTRACT

An air preheater, including a flue; an air channel; a first segment for producing secondary air; a second segment for producing primary air; a third segment for anti-condensation of ammonium bisulfate; and a fourth segment for curing of ammonium bisulfate. The flue and the air channel are disposed on the downstream of a denitration device. The first segment, the second segment, the third segment and the fourth segment are disposed in that order along the gas flow direction. The first segment, the second segment, the third segment and the fourth segment each include a phase-change heat exchanger. The phase-change heat exchanger includes a heat absorption segment disposed in the flue, a heat release segment disposed in the air channel, an ascending tube and a downcomer which are configured to connect the heat absorption segment and the heat release segment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of International Patent Application No. PCT/CN2014/075792 with an international filing date of Apr. 21, 2014, designating the United States, now pending, and further claims priority benefits to Chinese Patent Application No. 201410006896.4 filed Jan. 7, 2014. The contents of all of the aforementioned applications, including any intervening amendments thereto, are incorporated herein by reference. Inquiries from the public to applicants or assignees concerning this document or the related applications should be directed to: Matthias Scholl P.C., Attn.: Dr. Matthias Scholl Esq., 245 First Street, 18th Floor, and Cambridge, Mass. 02142.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an air preheater and a method for preventing corrosion and blockage of the same.

2. Description of the Related Art

Conventional flue gas denitration technology employs ammonia gas as a reducing agent. The reducing agent reacts with sulfur trioxide in the flue gas to yield ammonia bisulfate. Following the denitration process, an air preheater is provided to recycle heat energy.

Typically, the air preheater is of a regenerative type, as shown in FIG. 1, and includes a high temperature segment, a medium temperature segment, and a low temperature segment. Ammonium bisulfate exhibits strong viscosity in the medium temperature segment and the low temperature segment, and tends to adsorb dust particles and, as a result, block the air preheater. This increases the gas flow resistance, lowers the heat transfer efficiency, and adversely affects operation of the air preheater.

SUMMARY OF THE INVENTION

In view of the above-described problems, it is one objective of the invention to provide an air preheater that has good corrosion resistant property. The air preheater is particularly practicable following a selective catalytic reduction (SCR) denitration.

To achieve the above objective, in accordance with one embodiment of the invention, there is provided an air preheater, comprising a flue and an air channel disposed on a downstream of a denitration device, a first segment for producing secondary air, a second segment for producing primary air, a third segment for anti-condensation of ammonium bisulfate, and a fourth segment for curing of ammonium bisulfate. The first segment for producing secondary air, the second segment for producing primary air, the third segment for anti-condensation of ammonium bisulfate and the fourth segment for curing of ammonium bisulfate are disposed in that order along a gas flow direction. The second segment for producing primary air, the third segment for anti-condensation of ammonium bisulfate, and the fourth segment for curing of ammonium bisulfate each comprise a phase-change heat exchanger. The phase-change heat exchanger comprises a heat absorption segment disposed in the flue, a heat release segment disposed in the air channel, an ascending tube and a downcomer which are configured to connect the heat absorption segment and the heat release segment. The heat release segment is disposed higher than the heat absorption segment, and the phase-change heat exchangers are provided with a cycling medium. A wall surface temperature of the third segment for anti-condensation of ammonium bisulfate is higher than a dew temperature of the ammonium bisulfate. A wall surface temperature of the fourth segment for curing of ammonium bisulfate is lower than a solidification point temperature of the ammonium bisulfate.

In a class of this embodiment, the air preheater further comprises an acid-dew resistant segment disposed on a downstream of the fourth segment for curing of ammonium bisulfate. The acid-dew resistant segment comprises the phase-change heat exchanger, and a wall surface temperature of the acid-dew resistant segment is higher than an acid dew point temperature.

Preferably, the heat absorption segment in the second segment for producing primary air, the heat absorption segment in the third segment for anti-condensation of ammonium bisulfate, the heat absorption segment in the fourth segment for curing of ammonium bisulfate, the heat absorption segment in the acid-dew resistant segment are provided with a temperature sensor.

In a class of this embodiment, the cycling medium in the second segment for producing primary air, the third segment for anti-condensation of ammonium bisulfate, the fourth segment for curing of ammonium bisulfate, and the acid-dew resistant segment is water, Freon, or heat transfer oil.

Preferably, the first segment for producing secondary air is a high temperature segment of a regenerative air preheater.

Preferably, the first segment for producing secondary air comprises the phase-change heat exchanger, and the cycling medium in the second segment for producing primary air is water, Freon, or heat transfer oil.

Advantages of the air preheater according to embodiments of the invention are summarized as follows:

In the air preheater, the reaction of the escaped ammonia and the sulfur trioxide only produces gaseous and solid ammonium bisulfate, and no liquid ammonium bisulfate is produced, thus the air preheater is effectively prevented from blockage and corrosion caused by liquid ammonium bisulfate. Therefore, the service life of the air preheater is prolonged; the heat transfer efficiency of the boiler is improved; thus ensuring the stable and safe operation of the machine set.

It is another objective of the invention to provide a method for preventing the air preheater from corrosion and blockage. The air preheater is disposed on a downstream of a denitration device using SCR denitration technology. The method can prevent the corrosion and blockage of the heating surface of the air preheater.

To achieve the above objective, in accordance with another embodiment of the invention, there is provided a method for preventing an air preheater from corrosion and blockage, the method comprising:

-   -   a) introducing flue gas in a flue to an upstream of the air         preheater, and allowing the flue gas to pass a first segment for         producing secondary air, a second segment for producing primary         air, a third segment for anti-condensation of ammonium bisulfate         and a fourth segment for curing of ammonium bisulfate in that         order; introducing air in an air channel to a downstream of the         air preheater, and allowing the air to pass the fourth segment         for curing of ammonium bisulfate, the third segment for         anti-condensation of ammonium bisulfate, the second segment for         producing primary air, and the first segment for producing         secondary air;     -   b) adjusting a wall surface temperature of the third segment for         anti-condensation of ammonium bisulfate to be higher than a dew         temperature of the ammonium bisulfate, and adjusting a wall         surface temperature of the fourth segment for curing of ammonium         bisulfate to be lower than a solidification point temperature of         the ammonium bisulfate;     -   c) gasifying the ammonium bisulfate in the flue prior to and in         the third segment for anti-condensation of ammonium bisulfate;         curing the ammonium bisulfate in and after the fourth segment         for curing of ammonium bisulfate, the ammonium bisulfate being         cured on a wall surface of a pipe of the fourth segment and a         wall surface of a downstream pipe of the fourth segment; and     -   d) eliminating solid ammonium bisulfate on the wall surface of         the pipe of the fourth segment and on the wall surface of the         downstream pipe of the fourth segment.

Advantages of the method according to embodiments of the invention are summarized as follows:

Using the method, the wall surface temperature of the third segment and the fourth segment can be accurately controlled, so that the ammonium bisulfate is cured on the wall surface of the pipe of the fourth segment and the wall surface of the downstream pipe of the fourth segment, then the solid ammonium bisulfate is eliminated by blowing equipment. As a result, the air preheater is effectively prevented from blockage and corrosion, thus ensuring the safe, reliable, and stable operation of the boiler.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described hereinbelow with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an existing air preheater;

FIG. 2 is a schematic diagram of an air preheater in accordance with one embodiment of the invention; and

FIG. 3 is a schematic diagram of an air preheater in accordance with another embodiment of the invention.

In the drawings, the following reference numbers are used: 1. Hue; 2. Air channel; 3. First segment for producing secondary air; 4. Second segment for producing primary air; 5. Third segment for anti-condensation of ammonium bisulfate; 6. Fourth segment for curing of ammonium bisulfate; 7. Heat absorption segment; 8. Heat release segment; 9. Ascending tube; 10. Downcomer; 11. Acid-dew resistant segment; and 12. Temperature sensor.

DETAILED DESCRIPTION OF THE EMBODIMENTS

For further illustrating the invention, experiments detailing an air preheater and a method for preventing corrosion and blockage of the same after selective catalytic reduction (SCR) denitration are described below. It should be noted that the following examples are intended to describe and not to limit the invention.

As shown in FIG. 2, an air preheater comprises a flue 1 and an air channel 2 disposed on the downstream of a denitration device, a first segment for producing secondary air 3, a second segment for producing primary air 4, a third segment for anti-condensation of ammonium bisulfate 5, and a fourth segment for curing of ammonium bisulfate 6 disposed in that order along a gas flow direction. The second segment for producing primary air 4, the third segment for anti-condensation of ammonium bisulfate 5, and the fourth segment for curing of ammonium bisulfate 6 each comprise a phase-change heat exchanger. The phase-change heat exchanger comprises a heat absorption segment 7 disposed in the flue 1, a heat release segment 8 disposed in the air channel 2, an ascending tube 9 and a downcomer 10 which are configured to connect the heat absorption segment 7 and the heat release segment 8. The heat release segment 8 is higher than the corresponding heat absorption segment 7, and the phase-change heat exchanger is provided with a cycling medium. A wall surface temperature of the third segment for anti-condensation of ammonium bisulfate 5 is higher than a dew temperature of the ammonium bisulfate. A wall surface temperature of the fourth segment for curing of ammonium bisulfate 6 is lower than a solidification point temperature of the ammonium bisulfate.

High-temperature flue gas (between 350 and 440° C.) in the flue 1 enters an upstream of the assembled air preheater, and passes the first segment for producing secondary air 3, the heat-absorbing segment 7 of the second segment for producing primary air 4, the heat-absorbing segment 7 of the third segment for anti-condensation of ammonium bisulfate 5, and the heat-absorbing segment 7 of the fourth segment for curing of ammonium bisulfate 6. The high-temperature flue gas releases heat when passing the air preheater and heats the cycling medium in the heat absorption segment 7 of all phase-change heat exchangers. The cycling medium in all phase-change heat exchangers absorbs heat released by the flue gas and generates a lift force because of a density difference, thus the cycling medium in the heat absorption segment 7 enters the heat release segment 8 via the ascending tube 9, and after releasing heat to the air, the cycling medium in the heat release segment 8 is back to the heat absorption segment 7 via the downcomer 10, hereby a self-cycle is realized and no external power is needed. Because the pipe wall temperature of the third segment for anti-condensation of ammonium bisulfate 5 and the pipe wall temperature of an upstream of the third segment for anti-condensation of ammonium bisulfate 5 are higher than the dew temperature of the ammonium bisulfate (between 0 and 200° C.), and the pipe wall temperature of the fourth segment for curing of ammonium bisulfate 6 and the pipe wall temperature of a downstream of the fourth segment for curing of ammonium bisulfate 6 are lower than the solidification point temperature (147° C.) of the ammonium bisulfate, the reaction of escaped ammonia and sulfur trioxide only generates gaseous and solid ammonium bisulfate in the air preheater, no liquid ammonium bisulfate is produced, and the air preheater is effectively prevented from blockage and corrosion caused by liquid ammonium bisulfate. Therefore, a flue gas temperature is decreased; an air leakage is reduced; a service life of the air preheater is prolonged; a heat transfer efficiency of a boiler is improved; and finally a stable and safe operation of a machine set is ensured. Preferably, cycling media in the second segment for producing primary air 4, the third segment for anti-condensation of ammonium bisulfate 5, the fourth segment for curing of ammonium bisulfate 6, the acid-dew resistant segment 11 is selected from water, Freon, or heat transfer oil. In actual operation, a suitable cycling medium solution is determined according to different temperatures.

Preferably, when the pipe wall temperature of the downstream of the fourth segment for curing of ammonium bisulfate 6 is too low, an acid dew corrosion happens, thus along the flue gas flow direction, the downstream of the fourth segment for curing of ammonium bisulfate 6 is provided with an acid-dew resistant segment 11. The acid-dew resistant segment 11 comprises the phase-change heat exchanger, comprising the heat absorption segment 7, the heat release segment 8, the ascending tube 9 and the downcomer 10. The wall surface temperature of the acid-dew resistant segment 11 is higher than an acid dew point temperature (between 0 and 100° C.), thereby effectively preventing the acid dew corrosion, decreasing an energy consumption of the boiler, and saving energy and reducing emission to the largest extent.

In addition, the air is divided into two paths after heated by the second segment for producing primary air 4, as shown in FIGS. 2-3, the first path is used as a primary air for a pulverizing system, and the second path is heated by the first segment for producing secondary air 3 and is used as secondary air of the boiler, which means the primary air is not heated by the first segment for producing secondary air 3. Heat saved by the primary air can be used to heat materials in the third segment for anti-condensation of ammonium bisulfate 5, the fourth segment for curing of ammonium bisulfate 6, the acid-dew resistant segment 11, or a combination thereof.

Prior to starting the assembled air preheater, the air in the acid-dew resistant segment 11, the fourth segment for curing of ammonium bisulfate 6, the third segment for anti-condensation of ammonium bisulfate 5, and the second segment for producing primary air 4 is exhausted, so that no non-condensable gas exists in cycling pipes of the phase-change heat exchangers, and the blockage caused by water or gas is avoided. When the boiler is operated at a rated load, the flue gas temperature after denitration is between 350 and 440° C. and is gradually stable, and water volumes in the cycling pipes of the acid-dew resistant segment 11, the fourth segment for curing of ammonium bisulfate 6, the third segment for anti-condensation of ammonium bisulfate 5, and the second segment for producing primary air 4 are adjusted according to the flue gas temperature after denitration, a temperature of the primary air, and a temperature of the secondary air; correspondingly, steam pressures in the phase-change heat exchangers are adjusted and determined, then saturation temperatures are determined, and wall surface temperatures of the segments are determined. Preferably, the heat absorption segment 7 in the second segment for producing primary air 4, the heat absorption segment 7 in the third segment for anti-condensation of ammonium bisulfate 5, the heat absorption segment 7 in the fourth segment for curing of ammonium bisulfate 6, the heat absorption segment 7 in the acid-dew resistant segment 11 are provided with a temperature sensor 12 adapted to detect the wall surface temperatures of the segments.

Preferably, in the example, as shown in FIG. 2, the first segment for producing secondary air 3 is a high temperature segment of a regenerative air preheater. Optionally, the first segment for producing secondary air 3 can also be a phase-change heat exchanger, as shown in FIG. 3, the first segment for producing secondary air 3 also comprises the heat absorption segment 7, the heat release segment 8, the ascending tube 9, and the downcomer 10, and the cycling medium in the first segment for producing secondary air 3 is water, Freon, or heat transfer oil.

A method for preventing the air preheater from corrosion and blockage after an SCR denitration is also provided in the invention, and the method comprises:

-   -   a) introducing flue gas from the flue 1 to an upstream of the         air preheater, and allowing the flue gas to pass the first         segment for producing secondary air 3, the second segment for         producing primary air 4, the third segment for anti-condensation         of ammonium bisulfate 5, and the fourth segment for curing of         ammonium bisulfate 6 in that order; introducing air from the air         channel to a downstream of the air preheater, and allowing the         air to pass the fourth segment for curing of ammonium bisulfate         6, the third segment for anti-condensation of ammonium bisulfate         5, the second segment for producing primary air 4, and the first         segment for producing secondary air 3;     -   b) adjusting the wall surface temperature of the third segment         for anti-condensation of ammonium bisulfate 5 to be higher than         a dew temperature of the ammonium bisulfate, and adjusting the         wall surface temperature of the fourth segment for curing of         ammonium bisulfate 6 to be lower than a solidification point         temperature of the ammonium bisulfate;     -   c) gasifying the ammonium bisulfate in the flue 1 prior to and         in the third segment for anti-condensation of ammonium bisulfate         and an upstream of the third segment for anti-condensation of         ammonium bisulfate; curing the ammonium bisulfate in and after         the fourth segment for curing of ammonium bisulfate 6, the         ammonium bisulfate being cured on a wall surface of a pipe of         the fourth segment and a wall surface of a downstream pipe of         the fourth segment; and     -   d) eliminating solid ammonium bisulfate on the wall surface of         the pipe of the fourth segment and on the wall surface of the         downstream pipe of the fourth segment 6.

Advantages of the air preheater and the method are summarized as follows:

-   -   1. Using the method, the wall surface temperature of the third         segment and the fourth segment can be accurately controlled, so         that the ammonium bisulfate is cured on the wall surface of a         heat exchanger, meanwhile avoiding condensation of the ammonium         bisulfate on the wall surface of the heat exchanger; then the         solid ammonium bisulfate is eliminated by the blowing equipment.         As a result, the air preheater is effectively prevented from         blockage and corrosion, thus ensuring the safe, reliable, and         stable operation of the boiler, and the corrosion caused by the         ammonium bisulfate in a medium temperature segment and a low         temperature segment of the regenerative air preheater is         completely prevented.     -   2. The acid-dew resistant segment 11 is provided, and the wall         surface temperature of the acid-dew resistant segment 11 is         controlled so as to prevent the acid dew corrosion, decrease the         energy consumption of the boiler, and save energy and reduce         emission to the largest extent.     -   3. The phase-change heat exchanger is employed, and the flue gas         temperature is decreased to a lower level than the flue gas         temperature of the regenerative air preheater.     -   4. The air is directly divided into two paths following the         second segment for producing primary air 4, and the primary air         does not enter the first segment for producing secondary air 3,         thus the heat is saved.     -   5. The air leakage problem of the regenerative air preheater in         the prior art is solved by using the phase-change heat         exchanger.

Therefore, the phase-change heat exchanger compensates for the shortcomings in the prior art, featuring high industrial value.

Unless otherwise indicated, the numerical ranges involved in the invention include the end values. While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention. 

The invention claimed is:
 1. An air preheater, comprising: a flue; an air channel; a first segment for producing secondary air; a second segment for producing primary air; a third segment for anti-condensation of ammonium bisulfate; and a fourth segment for curing of ammonium bisulfate; wherein the flue and the air channel are disposed on a downstream of a denitration device; the first segment for producing secondary air, the second segment for producing primary air, the third segment for anti-condensation of ammonium bisulfate and the fourth segment for curing of ammonium bisulfate are disposed in that order along a gas flow direction; the second segment for producing primary air, the third segment for anti-condensation of ammonium bisulfate, and the fourth segment for curing of ammonium bisulfate each comprise a phase-change heat exchanger; the phase-change heat exchanger comprises a heat absorption segment disposed in the flue, a heat release segment disposed in the air channel, an ascending tube and a downcomer which are configured to connect the heat absorption segment and the heat release segment; the heat release segment is disposed higher than the heat absorption segment, and the phase-change heat exchanger is provided with a cycling medium; and a wall surface temperature of the third segment for anti-condensation of ammonium bisulfate is higher than a dew temperature of the ammonium bisulfate; a wall surface temperature of the fourth segment for curing of ammonium bisulfate is lower than a solidification point temperature of the ammonium bisulfate.
 2. The air preheater of claim 1, wherein the air preheater further comprises an acid-dew resistant segment disposed on a downstream of the fourth segment; the acid-dew resistant segment also comprises the phase-change heat exchanger, and a wall surface temperature of the acid-dew resistant segment is higher than an acid dew point temperature.
 3. The air preheater of claim 2, wherein the heat absorption segment in the second segment for producing primary air, the heat absorption segment in the third segment for anti-condensation of ammonium bisulfate, the heat absorption segment in the fourth segment for curing of ammonium bisulfate, the heat absorption segment in the acid-dew resistant segment each are provided with a temperature sensor.
 4. The air preheater of claim 2, wherein the cycling medium in the second segment, in the third segment, in the fourth segment, and in the acid-dew resistant segment is water, Freon, or heat transfer oil.
 5. The air preheater of claim 1, wherein the first segment for producing secondary air is a segment of a regenerative air preheater.
 6. The air preheater of claim 1, wherein the first segment for producing secondary air comprises the phase-change heat exchanger, and the cycling medium in the second segment for producing primary air is water, Freon, or heat transfer oil.
 7. A method for preventing corrosion and blockage of an air preheater, the air preheater being disposed on a downstream of a denitration device, the method comprising: 1) introducing flue gas in a flue to an upstream of the air preheater, and allowing the flue gas to pass a first segment for producing secondary air, a second segment for producing primary air, a third segment for anti-condensation of ammonium bisulfate and a fourth segment for curing of ammonium bisulfate in that order; introducing air in an air channel to a downstream of the air preheater, and allowing the air to pass the fourth segment for curing of ammonium bisulfate, the third segment for anti-condensation of ammonium bisulfate, the second segment for producing primary air, and the first segment for producing secondary air; 2) adjusting a wall surface temperature of the third segment for anti-condensation of ammonium bisulfate to be higher than a dew temperature of the ammonium bisulfate, and adjusting a wall surface temperature of the fourth segment for curing of ammonium bisulfate to be lower than a solidification point temperature of the ammonium bisulfate; 3) gasifying the ammonium bisulfate in the flue prior to and in the third segment for anti-condensation of ammonium bisulfate; curing the ammonium bisulfate in and after the fourth segment for curing of ammonium bisulfate, the ammonium bisulfate being cured on a wall surface of a pipe of the fourth segment and a wall surface of a downstream pipe of the fourth segment; and 4) eliminating solid ammonium bisulfate on the wall surface of the pipe of the fourth segment and on the wall surface of the downstream pipe of the fourth segment. 